1. Field of Invention
This invention generally relates to identifying desired features in a crystal. In particular, the invention relates to a method for identifying desired features in an offorientation crystal using radiation emitted in directions transverse to each other relative to the crystal.
2. Description of Related Art
Single-crystal semiconductor wafers used to produce integrated circuits (ICs) and the like are typically cut from a monocrystalline semiconductor ingot. The semiconductor ingot is typically grown using the Czochralski method whereby a seed crystal is dipped into a semiconductor melt and withdrawn from the melt. As the seed crystal is withdrawn from the semiconductor melt, the semiconductor melt crystallizes to form a roughly cylindrically shaped ingot.
Semiconductor chip manufacturers often require semiconductor wafers having different crystallographic orientations, such as &lt;100&gt; or &lt;111&gt;, which are well known to those of ordinary skill in the art. Since the semiconductor melt that grows from the seed crystal in the Czochralski method has the same crystallographic orientation as the seed, growing an ingot having a desired crystallographic orientation is done mainly by selecting an appropriate seed crystal. Once an ingot of a desired diameter and length has been grown, the end caps of the ingot are removed and the ingot is ground into a cylindrical shape in preparation for cutting wafers from the ingot.
Depending on specific manufacturing requirements, an orientation flat and/or a notch are ground into the ingot at a specific location. The orientation flat and/or notch indicate the relative location of specific crystallographic features, such as habit lines, or nodes, in the ingot that are important to manufacturing ICs and the like.
FIG. 1 shows a side view of a typical ingot 1 that has had its end caps removed and has been ground into a cylindrical shape. The ingot 1 has a &lt;100&gt; crystallographic orientation such that the (100) planes 2 are perpendicular to a longitudinal axis 3 of the ingot 1. Habit lines 4 indicate the crystal orientation and are at 90.degree. intervals around the ingot 1, as shown in an end view of the ingot 1 in FIG. 2.
The crystallographic planes in an ingot 1 are not visible to the naked eye. Therefore, an x-ray goniometer is typically used to determine where crystallographic planes are located in the ingot 1. As is well understood by those of skill in the art, the crystallographic planes are typically found by directing x-ray emission at the ingot 1 and detecting changes in reflection as the ingot 1 is moved. Once a desired crystallographic plane, e.g. a plane or planes associated with a desired habit line 4, is located, an orientation flat OF and/or notch can be ground into the ingot at a desired location, e.g. at an angle of 45.degree. to the habit line 4, as shown in FIG. 2.
Other types of semiconductor ingots, such as &lt;111&gt; ingots, have crystalline structures that require the use of an x-ray goniometer to first identify crystallographic planes in a radial direction relative to the ingot, and then confirm in an axial direction that one of the identified crystallographic plane(s) is acceptable for determining the position of an orientation flat and/or notch in an axial direction relative to the crystal. FIG. 3 shows a schematic end view of a &lt;111&gt; ingot 1. Three of the habit lines 4 (shown in solid lines) are acceptable for orientation flat or other marking location. Three other habit lines (shown in dashed lines) are not acceptable for determining a marking location.
Therefore, the ingot 1 is first illuminated with x-rays in a radial direction to identify (110) planes associated with two habit lines 4. Then, the ingot 1 is illuminated with x-rays in an axial direction. By x-raying in the axial direction, at least one (440) plane associated with one habit line 4 that is acceptable for determining a marking location can be identified. Habit lines 4 that are not appropriate for identifying a marking location do not have an associated detectable (440) plane. Thus, a desired key growth line can be identified, and the ingot 1 appropriately marked with an orientation flat and/or groove or other marking.
Typically, &lt;111&gt; ingots 1 are inspected using two different x-ray machines, i.e. a first x-ray machine is used to inspect the ingot 1 in a radial direction, and the ingot 1 is moved to a second x-ray machine and inspected in an axial direction. A dual x-ray machine has been proposed for inspecting semiconductor ingots that is capable of inspecting an ingot in both the axial and radial directions without removing the ingot 1 from the machine. One type of dual x-ray device is discussed in more detail below in connection with FIG. 7.
In contrast to the on-orientation &lt;100&gt; and &lt;111&gt; ingots discussed above, manufacturing requirements occasionally require that an ingot 1 be prepared off-orientation. For example, FIG. 4 shows a &lt;100&gt; 4.degree. off ingot 1. In contrast to the standard orientation crystal shown in FIG. 1, the 4.degree. off ingot 1 shown in FIG. 4 has its (100) planes 2 at an angle of 4.degree. to a line perpendicular to a longitudinal axis 3 of the ingot 1. As shown in FIG. 5, the habit lines 4 are not evenly spaced around the ingot 1 as in the standard &lt;100&gt; ingot 1. Instead, two of the habit lines 4 are at an 88.degree. angle to each other, and two other habit lines 4 are at a 92.degree. angle to each other.
To determine the location of a key growth line and appropriately mark an ingot 1, an operator typically visually inspects the ingot 1 before the ingot 1 is machined into a cylindrical shape to determine the approximate location of the key growth line. Then, a radial x-ray is used to confirm the precise location of the key growth line. An axial x-ray can be used to confirm that the proper key growth line has been identified. The ingot 1 is then marked with an orientation flat and/or groove or other marking to indicate the crystal feature orientation.